This invention relates to improved methods for the production of foraminous cathodes having a continuous coating of cation exchange copolymers, reinforced and unreinforced, useful as separators in batteries and fuel cells as well as electrochemical cells such as chlor-alkali cells.
Typical of the cation exchange copolymers involved in the instant invention are the fluorocarbon vinyl ether polymers disclosed in U.S. Pat. No. 3,282,875. This patent discloses the copolymerization of fluorocarbon vinyl ethers having sulfonyl groups attached thereto with fluorinated vinyl compounds. Of the various copolymers listed in U.S. Pat. No. 3,282,875 is the copolymer produced by the copolymerization of tetrafluoroethylene with perfluoro(3,6-dioxa-4-methyl-7-octene sulfonyl fluoride). This is the base copolymer from which most of the membranes in commercial use today are made from.
Another example of cation exchange resins useful in the instant invention are those described in U.S. Pat. No. 3,718,627. The disclosed ion exchange resins are copolymers of tetrafluoroethylene and compounds of the formula CF.sub.2 =CF(CF.sub.2).sub.n SO.sub.2 F.
After polymerization of either of these materials of the prior art, the copolymer must be hydrolyzed to obtain its ion exchange character. Typically, such materials are treated with caustic to convert the sulfonyl halide group to the alkali metal salt thereof.
These known perfluorocarbon-type cation exchange membranes containing only sulfonic acid groups, however, have been found to have a disadvantage that when used in the electrolysis of an aqueous solution of an alkali metal halide, they tend to permit penetration there through of excessive hydroxyl ions by back migration from the cathode compartment because of the high hydrophilicity of the sulfonic acid group. As a result, the current efficiency during electrolysis at higher caustic concentrations is lower. At extremely high caustic concentrations, the process becomes economically disadvantageous compared to other methods of electrolysis of sodium chloride solutions, such as the mercury or diaphragm process. Many attempts have been made to avoid this disadvantage of lower current efficiency by a number of means. Initially, people in the art attempted to utilize membrane containing less sulfonic acid groups, or expressed in another manner, membrane material having a higher equivalent weight. Such lowering of the sulfonic acid group concentration or the increase of the equivalent weight of the membrane does indeed limit the back migration of hydroxyl ions, but results in a serious decrease in the electroconductivity of the membrane and thus, a proportional increase in the power consumption is noted.
A number of solutions of this problem have been attempted in the prior art. Typical of such attempts is the surface modification of the membrane material of the cathode side to attempt to minimize back migration of hydroxyl ions. One such attempt was to laminate to the surface of a membrane of low equivalent weight a thin surface layer of material having a higher equivalent weight so as to minimize back migration. This attempt has not been successful due to the fact that such laminated membranes do not joint together well and in operation tend to separate and, in extreme cases, rupture. The laminating technique itself puts much stress on the copolymeric materials in that higher temperatures are required in the calendering of the melt processable copolymer to thin sheets. While the copolymeric material is melt processable, the temperatures at which it flows are very close to the temperatures at which degradation can take place. Thus, melt processable fabrication methods must be tightly controlled and are at best difficult.
Later attempts to improve membrane cells by reducing hydroxyl back migration in, for example, chlor-alkali cells, was to treat the cathode surface of the membrane with an amine whether mono- or diamine or ammonia. Also, to surface modify a sulfonyl membrane to convert the surface layer facing the cathode to the corresponding carboxylic material. Typical of this method is that described in U.S. Pat. No. 4,151,053, incorporated herein by reference.
The manufacture of thin sheets of the copolymeric materials of the instant invention in the past have been as expressed previously very tedious. The copolymeric material would be melted and calendered of the required thickness. In cases where reinforcing fabric was included with the sheet of membrane, the problems were further increased because the flowability of the copolymeric material at processing temperatures is limited and if the temperatures are raised further to improve the flowability, the polymeric material degrades. In almost all cases, the membrane materials must be reinforced so as to be sufficiently rugged to be economically advantageous in the uses envisioned. Typical of the problems encountered in preparing fabric reinforced sheet membranes can be found in U.S. Pat. No. 4,147,844. In the case of membranes deposited directly on the cathode or material which is directly on the cathode, further processing problems existed using melt techniques. Among the most persistent problem was poor adhesion of the membrane to the cathode surface. Typical of such prior art methods are those disclosed in U.S. Pat. No. 4,036,728 wherein diaphragm type electrolytic cells are converted to membrane electrolytic cells by depositing membrane on the cathode.